java - System.in.read() behaviour i can't explain

Question

class E92StringDemo {

    public static void main(String args[]) throws java.io.IOException {

        String strObj1 = "First String";

        for (int i = 0; i < strObj1.length(); i++) {

            System.out.print(strObj1.charAt(i));
            System.in.read(); //just to pause the execution till i press enter key

        }
    }
}

I want the output to come like:

F
i
r
s
t...

but the output is coming like:

F
ir
st
S
tr
in
g

I am not sure how come 2 characters are getting displayed in one line with every press of an enter key( )?

I am running windows 8 and using a command prompt to run the file using javac.

Answer 1

Problem

System.in.read() only holds execution of your application if there is no data to read in standard input stream (represented by System.in).

But in console when you press ENTER, two things happen:

• console cursor is moved to next line

• OS dependent line separator* is placed in standard input, which for Windows is :

  • carriage return - placed at index 13 in Unicode Table
  • line feed - placed at index 10 in Unicode Table

So as you see if you want to pause your loop in each next iteration, you will need to empty data from input stream before leaving current iteration. But System.in.read() reads only one character at a time, in your case leaving for next iteration (so no pause there).

So before pause will be again available you need to read twice in one iteration.

Solution

If you want to get rid of this problem in OS independent way use BufferedReader#readLine() or Scanner#nextLine like:

String strObj1 = "First String";
try(Scanner sc = new Scanner(System.in)){//will automatically close resource
    for (int i = 0; i < strObj1.length(); i++) {
        System.out.print(strObj1.charAt(i));
        sc.nextLine();
    }
}

These methods also solve problem of potential extra characters placed before pressing enter, since each of them will also be placed in standard input stream, which would require additional .read() calls.

---

* along with rest of potential characters which ware provided before pressing enter